A color filter of a liquid crystal display device has a red pixel area, a green pixel area, and a blue pixel area. All of these pixel areas have a structure, in which a thin film made of a synthetic resin having an organic pigment dispersed therein is provided on a substrate, and as the organic pigment, organic pigments of each color including red, green, and blue are used.
Among these pixel areas, as a blue organic pigment for forming the blue pixel area, in general, an ε-type copper phthalocyanine pigment (C.I. pigment blue 15:6) is used, and if necessary, in order to mix colors, a small amount of a dioxazine violet pigment (C.I. pigment violet 23) of a violet organic pigment or a violet dye is used in combination.
The organic pigments used when producing a color filter are required to have properties which are totally different from the conventional organic pigments used for general purpose. Specifically, the organic pigments are required to cause a display screen of the liquid crystal display device to be brighter (increase brightness) or, in the same manner, to cause a display screen to be seen much clearly (increase contrast). Furthermore, since a color filter is exposed to the temperature of 200° C. or higher because of film attachment of transparent electrodes or alignment film attachment of polyimide in the step after producing a color filter, a pigment for color filters which has excellent heat resistance and satisfies the above-mentioned properties has been reviewed.
Meanwhile, a phthalocyanine pigment similar to the present invention is disclosed, for example, in the following documents.
PTL 1 discloses a method for producing a pigment composition which includes an ε-type phthalocyanine pigment and a phthalocyanine pigment derivative being substituted with a phthalimide methyl group with the number of the substituents being 1 to 4; however, a distribution of the substituents of the pigment derivative is not mentioned, and heat resistance is not sufficient to satisfy the heat resistance demanded in recent years.
In the same manner, PTL 2 and PTL 3 suggest a pigment composition for color filters which includes a fine ε-type copper phthalocyanine pigment and a phthalocyanine pigment derivative which has been substituted with a phthalimide methyl group with the number of the substituents being 1 to 4, and a method for producing the pigment composition by solvent salt milling. However, in the same manner as PTL 1, a distribution of the substituent of the pigment derivative is not mentioned and the inventions of PTL 2 and PTL 3 are insufficient for obtaining desired heat resistance.
However, when an organic pigment composition which includes a pigment derivative in which the ratio of a multi-substituted phthalimide alkyl group to the whole amount of the pigment derivative is specified as in the present invention and a phthalocyanine pigment is used for a color filter, brightness is less decreased and high heat resistance is obtained, even if the organic pigment composition undergoes a thermal history at high temperature in a step of producing a color filter.